Separator for sealed lead acid battery

ABSTRACT

A separator for a valve-regulated lead acid battery consists mainly of fine glass fibers. The separator includes inorganic powder in an amount of 5 to 30% by weight and natural pulp in an amount of 3 to 20% by weight, and has a density of not less than 0.165 g/cm 3 . The separator sufficiently suppresses the occurrence of electrical short circuits between the positive and negative electrode plates.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation application of PCT/JP99/07091 filed on Dec. 16,1999.

FIELD OF THE INVENTION

The present invention relates to a separator for a valve-regulated leadacid battery. More particularly, it relates to a separator for avalve-regulated lead acid battery, which is mainly made of fine glassfibers and also includes inorganic powder and natural pulp in order toimprove its property of suppressing short circuits between positive andnegative electrode plates of the battery.

BACKGROUND OF THE INVENTION

A conventional separator for a valve-regulated lead acid battery has ashape of a sheet, and contains glass fibers as the main component. Aconventional separator is occasionally penetrated or ripped by localpressure given from a salience on an electrode plate or by crystalgrowth of lead in the separator during charging thereof, which causes ashort circuit between positive and negative electrode plates andinterrupts electrical charge or discharge of the battery. This isbecause the conventional separator does not have a sufficient mechanicalstrength, so that it is easily penetrated or ripped by the salience onthe electrode plate, and because the conventional separator has arelatively low density and contains relatively large pores to easilyallow crystal growth. With a thinner conventional separator, a shortcircuit occurs more often because the decrease of the thickness makesthe mechanical strength of the separator weaker and the positive andnegative electrode plates closer.

Increasing density and decreasing pore size and/or pore volume in aseparator may prevent crystal growth of lead and also prevent anelectrical short circuit. It is one possible way to increase the densityand decrease the pore size and/or pore volume of the separator that theseparator includes particles of inorganic powder such as silica powderbetween the glass fibers. However, when the separator includes theinorganic powder, the relative amount of the glass fibers becomes lessand interlacing of the glass fibers is reduced. As a result, themechanical strength of the separator is decreased, so that a shortcircuit caused by pressure from salience of the electrode plates becomeseasy to occur.

Increasing the mechanical strength of a separator can prevent anothercause, penetration or ripping of the separator. Japanese patentpublications S54-22531A, S56-99968A, and S58-663B describe to mixsynthetic fibers with glass fibers for strengthening the separator.However, since the synthetic fiber is less hydrophilic than the glassfiber, the separator including the synthetic fibers has a drawback oflower liquid absorbency and liquid retention for sulfuric acid solution.

Japanese patent publication S64-52375A discloses a separator includingbeaten cellulose to improve its mechanical strength without lowering itsliquid absorbency or liquid retention. However, beaten cellulose bringsonly a slight change in the density of the separator so that it does notprevent short circuits caused by crystal growth of lead.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to overcome disadvantagesdescribed above and to provide a separator for a valve-regulated leadacid battery which is mainly made of fine glass fibers and also includesinorganic powder and natural pulp in order to improve the property ofsuppressing the short circuits between the positive and negativeelectrode plates of the battery, and, further, which has an increaseddensity to make the separator thinner and applicable to flat electrodeplates.

A separator for a valve-regulated lead acid battery of the presentinvention is mainly made of fine glass fibers and includes inorganicpowder and beaten natural pulp. The separator contains 5 to 30% byweight of the inorganic powder and 3 to 20% by weight of the naturalpulp. The density of the separator is equal to or more than 0.165 g/cm³.

An electrical short circuit between positive and negative electrodeplates in a valve-regulated lead acid battery occurs mostly due to thefollowing two causes;

[1] Mechanical Short Circuit

A salience on an electrode plate (unevenness of a grid, granule ofactive material, and the like) gives local pressure or shearing force toa separator. When the separator is not strong enough, the saliencepenetrates or rips the separator and reaches the opposite plate to causea short circuit.

[2] Electrochemical Short Circuit

At the end of electrical discharging of the battery, sulfate ions inelectrolyte are consumed and the electrolyte becomes almost pure water,whereby the electrolyte is increased in its solubility of the lead ion.Consequently, a part of lead sulfate formed on the positive and negativeelectrode dissolves into the electrolyte. When the battery is chargedafter that, lead ions in the electrolyte are reduced on the negativeplate and lead is deposited thereon and crystallized. The lead crystalgrows into the separator until it reaches the opposite plate to cause ashort circuit.

In the present invention, in order to suppress the occurrence of shortcircuits due to the above cause [2], inorganic powder such as silicapowder is mixed into the separator to decrease the size of pores in theseparator and increase the density of the separator. Furthermore, beatennatural pulp is mixed into the separator to improve the tensile strengthand the durability to penetration so that short circuits due to theabove cause [1] is suppressed. Since both inorganic powder and naturalpulp are highly hydrophilic, the performance of the separator is notreduced.

The separator of the present invention suppresses effectively the shortcircuit due to the above two causes. The separator can be produced inone process, thereby lowering the production cost.

Furthermore, since the separator of the present invention has arelatively high density, it can be thinner and successfully applicableto flat electrode plates placed in a small distance from 0.3 to 0.7 mm.

In the present invention, acid resistant glass fibers having a meanfiber diameter of not greater than 1 μm are suitable for the fine glassfibers and the silica powder having a specific surface area of not lessthan 100 m²/g is suitable for the inorganic powder. Natural pulp to beused is preferably beaten to have the Canadian freeness of 250 mL orless.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail.

The separator for a valve-regulated lead acid battery of the presentinvention is mainly made of fine glass fibers, includes inorganic powderin an amount of 5 to 30% by weight and beaten natural pulp in an amountof 3 to 20% by weight, and has a density of 0.165 g/cm³ or higher.

The fine glass fibers are preferably acid resistant glass fibers, morepreferably alkali-contained glass fibers with good acid resistance andhaving a mean fiber diameter equal to or smaller than 1 μm. Theseparator includes the fine glass fibers preferably in a range from 50to 92% by weight. Fine glass fibers with a mean fiber diameter largerthan 1 μm have lower liquid retention and sheet-formability. When theseparator includes the fine glass fibers in an amount of less than 50%by weight, the separator does not have a good liquid retention. Morethan 92% by weight of the fine glass fibers results in relatively smallamounts of inorganic powder and natural pulp, whereby the separator isnot fully prevented from short circuits.

When the amount of the inorganic powder is less than 5% by weight, theoccurrence of short circuits is not suppressed successfully. More than30% by weight of the inorganic powder means relatively small amounts ofthe fine glass fiber and the natural pulp, whereby the separator doesnot have enough high mechanical strength. Therefore, the amount ofinorganic powder is preferably in a range from 5 to 30% by weight.

Silica, titanium dioxide and diatomaceous earth can be used as theinorganic powder. Silica powder having a specific surface area equal to100 m²/g or larger is most suitable because of its high hydrophilicproperty and low cost. Silica powder having a specific surface areaequals to 100 m²/g or more have enough pores inside and on surfacesthereof so as to give sufficient liquid retention to the separator.

When the separator includes beaten natural pulp of less than 3% byweight, the separator does not have enough high mechanical strengthsince the separator includes the inorganic powder which reduces themechanical strength of the separator, whereby the separator is notprevented from short circuits fully. While, when the amount of beatennatural pulp exceeds 20% by weight, the separator becomes too hard tomaintain the adhesion with electrode plates. Therefore, beaten naturalpulp is preferably included in the separator in an amount of 3 to 20% byweight.

Among the natural pulp, it is suitable to use soft wood pulp beaten by abeater or the like. The soft wood pulp has long fiber length and ishomogeneous so that it can be very effective in reinforcing theseparator. The soft wood pulp is preferably beaten to the extent of 250mL or less in the Canadian freeness (the Canadian standard freeness),more preferably 150 mL or less, at a concentration of 0.3% by weight.(For reference, freeness of unbeaten natural pulp is 600 mL or more.)Natural pulp beaten to the extent of such freeness has a very largespecific surface area and pore volume, several times as large as thoseof ordinary pulp do. And it has good reactivity, hydrophilic property,liquid retention and acid resistance, and further, it works veryeffectively as reinforcement. Consequently, even a small amount of suchbeaten natural pulp is enough to improve strength and hardness of aseparator sufficiently, and that small amount of pulp cannot ruin liquidretention and liquid absorbency of the separator.

In the present invention, a part of natural pulp can be substituted byfibrillated cellulose, which is natural pulp finely divided into thesize ofmicrofibril and effective to improve mechanical strength of theseparator. When the amount of fibrillated cellulose substituted fornatural pulp is more than 5% by weight, the separator becomes too hardand thus loses its adhesive quality with the electrode plates.Therefore, the amount of the fibrillated cellulose should be not greaterthan 5% by weight and the total amount of fibrillated cellulose andbeaten natural pulp should be not greater than 20% by weight.

The separator of the present invention is produced by mixing the abovecompositions in the ratio that makes the density of the separator 0.165g/cm³ or more, preferably in a range 0.165-0.250 g/cm³, and, then,making sheet of the mixture.

In the present invention, when the density of the separator is lowerthan 0.165 g/cm³, the separator includes a lot of pores. That means sucha separator having the density of lower than 0.165 g/cm³ can notsufficiently suppress the short circuits when it is thin to be adaptedto flat electrode plates. While, when the separator has the density ofhigher than 0.250 g/cm³, liquid retention of the separator is lowered.Therefore, the separator has the density preferably in a range from0.165 to 0.250 g/cm³.

The separator of the present invention has such a high density that itprevents short circuits of the battery effectively, even when thethickness thereof is as small as 0.4 to 0.8 mm, wherein the thickness isdetermined by the method of measurement described later with regard toExamples.

The separator of the present invention is very useful for flat electrodeplates.

Hereinafter, the present invention will be described in detail withreference to examples and comparative examples. It is to be understoodthat the present invention is not limited to the following examples.

Materials used in the examples and comparative examples are given below.

Materials

Glass fiber: alkali-containing glass fiber having a mean fiber diameterof 0.8 μm.

Powder: silica powder having a specific surface area of about 200 m²/g.

Beaten natural pulp: soft wood pulp beaten to the extent of about 150 mLin the Canadian freeness.

Thermoplastic organic fiber: polyester fiber having a mean fiberdiameter of about 20 μm and a fiber length of about 5 m.

EXAMPLES 1 AND 2, AND COMPARATIVE EXAMPLES 1-4

Samples of separators for valve-regulated lead acid batteries wereprepared with compositions given in Table 1. Characteristics of thesamples were measured by the following methods and the results are givenin Table 1.

i) Thickness [mm] and Density [g/cm³]

Thickness T of each sample was measured under a pressure of 0.2 kgf/cm²(20 kPa) applied through the thickness thereof according to SBA4501.Density of each sample was calculated by the formula W/(T×S), where T isa thickness measured as the above, W is mass measured by an electronicbalance, and S is an area.

ii) Tensile strength [gf/10 mm²(N/10 mm²)]

Tensile strength of each sample was measured according to SBA4501.

iii) Strength against penetration

Strength of each sample against penetration was estimated by thefollowing method: to press a needle having a diameter of 1 mm and aspherical tip thereof vertically against the fixed sample at a speed of120 mm/min; and to measure a maximum load applied to the needle at themoment when the needle stuck the sample thereinto. Since the maximumload was easily affected by a slight difference in shapes of needletips, the measured maximum load was compared relatively with the maximumload of the standard sample (Comparative Example 1), which was set to100.

iv) Liquid absorbency [mm/min]

Liquid absorbency of each sample was measured in the following manner:immerse the bottom of a sample vertically into a dilute solution ofsulfuric acid having a specific gravity of 1.30; measure a rise of thesolution soaking into the sample in one minute after it was immersed.

v) Electrochemical short circuit time

A sample separator having 0.5 mm of thickness was disposed in betweentwo flat electrode plates with 7 mm² of area, and they were soaked intoa saturated solution of lead sulfate. Then, a constant voltage of 10Vwas applied to the sample under 0.3 kgf/m²(3 Pa) of pressure. Whenmetallic lead which grew from the negative plate and reached thepositive plate, electrical resistance between the electrode platesdropped remarkably. Measure the elapsed time from applying the voltageto the remarkable drop of the resistance. Divide the measured time bythe thickness of the separator to obtain a measured electrochemicalshort circuit time. The measured electrochemical short circuit time ofeach sample was compared with the result of the standard sample(Comparative Example 1), which was set to 100.

Among the above characteristics, strength against penetration is abarometer of the occurrence of mechanical short circuits. The stronger aseparator is against penetration, the more durable it is againstmechanical short circuits. The separator is better at preventingelectrochemical short circuits, if it has a longer electrochemical shortcircuit time.

TABLE 1 Examples Comparative Examples 1 2 1 2 3 4 Compositions (% byweight) Glass fiber 80 75 100 80 75 90 Silica powder 10 20 — — 20 —Beaten pulp 10 5 — — — 10 Thermoplastic organic fiber — — — 20 5 —Characteristics Thickness(mm) 0.5 0.5 0.5 0.5 0.5 0.5 Density(g/cm³⁾0.18 0.20 0.14 0.15 0.20 0.15 Tensile strength (gf/10mm²) 480 360 440660 300 720 (N/10mm²) 4.7 3.5 4.3 6.5 2.9 7.1 Strength againstpenetration* 190 120 100 270 70 200 Liquid absorbency(mm/min) 50 50 5040 45 50 Electrochemical short 4500 6000 100 250 1600 300 circuit time**: Relative values in setting results of Comparative example 1 to 100.

As shown in Table 1, the sample of Comparative Example 1 composed onlyof glass fibers is not strong against penetration and has a shortelectrochemical short circuit time. The sample of Comparative Example 2composed of glass fibers and organic fibers is relatively strong againstpenetration, yet has a relatively short electrochemical short circuittime and poor liquid absorbency. The sample of Comparative Example 3composed of glass fibers, silica powder and organic fibers has a longelectrochemical short circuit time, yet is very weak againstpenetration. The sample of Comparative Example 4 composed of glassfibers and beaten pulp is very strong against penetration and good atliquid absorbency, yet has a relatively short electrochemical shortcircuit time.

On the other hand, each sample of Examples 1 and 2 composed of glassfibers, silica powder and beaten pulp in the ratio given in Table 1 isvery strong against penetration and has a long electrochemical shortcircuit time without lowering liquid absorbency. Therefore, theseseparators are very good at preventing short circuits.

INDUSTRIAL APPLICABILITY

As detailed above, according to the present invention, the followingadvantages (1) to (4) are given to the separator, so that the separatorsufficiently suppresses the occurrence of short circuits betweenpositive and negative electrode plates and has excellent qualities as aseparator such as liquid absorbency and a low production cost. Theseparator of the present invention is exceedingly useful when it is madethin to use for a separator between flat electrode plates.

(1) The separator has high density because of inorganic powder retainedamong glass fibers. The inorganic powder in the pore of the separatorprevents crystal growth of metallic lead. Microfilament of beatennatural pulp works in the same way with the inorganic powder. As aresult, an electrical short circuit caused by crystal growth of metalliclead is suppressed.

(2) Beaten natural pulp makes an improvement in mechanical strength of aseparator, especially in strength against penetration. Therefore, theseparator cannot be easily penetrated nor ripped by local pressurecaused by salience of electrode plates.

(3) Since the separator consists of hydrophilic materials only, it hasan excellent hydrophilic property and liquid retention so that thebattery has an excellent performance.

(4) The separator can be easily produced through one process of mixingand making sheet to allow a low production cost.

What is claimed is:
 1. A separator for a valve-regulated lead acidbattery, which is mainly made of fine glass fibers and also includesinorganic powder and beaten natural pulp, wherein the amount of saidinorganic powder is 5 to 30% by weight, the amount of said natural pulpis 3 to 20% by weight, and the density of the separator is from 0.165g/cm³ to 0.250 g/cm³.
 2. A separator for a valve-regulated lead acidbattery as claimed in claim 1, wherein said fine glass fibers are acidresistant glass fibers having a mean fiber diameter equal to or smallerthan 1 μm.
 3. A separator for a valve-regulated lead acid battery asclaimed in claim 1, wherein said inorganic powder is at least oneselected from the group consisting of silica, titanium dioxide, anddiatomaceous earth.
 4. A separator for a valve-regulated lead acidbattery as claimed in claim 1, wherein said inorganic powder is silicapowder having a specific surface area equal to or larger than 100 m²/g.5. A separator for a valve-regulated lead acid battery as claimed inclaim 1, wherein said natural pulp is beaten to the extent of havingCanadian freeness equal to or lower than 250 ml.
 6. A separator for avalve-regulated lead acid battery as claimed in claim 1, wherein theamount of said fine glass fibers is 50 to 92% by weight.
 7. A separatorfor a valve-regulated lead acid battery as claimed in claim 1, whereinsaid natural pulp is soft wood pulp beaten by a beating device includinga beater.
 8. A separator for a valve-regulated lead acid battery asclaimed in claim 1, wherein a part of said natural pulp is fibrillatedcellulose.
 9. A separator for valve-regulated lead acid battery asclaimed in claim 8, wherein the amount of fibrillated cellulose is equalor less than 5% by weight, and the total amount of the fibrillatedcellulose and the beaten natural pulp is equal to or less than 20% byweight.
 10. A separator for a valve-regulated lead acid battery asclaimed in claim 1, wherein the density of the separator is 0.165 to0.250 g/cm³.
 11. A separator for a valve-regulated lead acid battery asclaimed in claim 1, wherein the thickness of the separator is 0.4 to 0.8mm.